In the production of explosives and propellants, one of the problems confronting manufacturers and users is the fact that the reactive portions of such compositions are not easily handled. For example, propellants or explosives are difficult to handle, pour, or cast due to their inherent physical and chemical properties, which can limit the usefulness of such propellants or explosives. For rocket propulsion, it is necessary to provide a propellant which can be easily placed within the rocket motor and which will stay in place until used. It is also desireable to have a mechanism to remove the propellant easily from the rocket motor prior to use.
In order to improve the physical and chemical characteristics of such compositions, it has become conventional to bind the reactive components within a composition which provides the overall the mixture with the necessary physical and chemical characteristics. One such composition has become known as a castable composite solid propellant. As the name implies, such a propellant can be cast into desired shapes, or can be cast into a particular operating environment, such as a rocket motor.
Typical castable solid propellant formulations include particulate and reactive solid materials. Such materials can, for example, include aluminum, ammonium perchlorate, and a burn rate catalyst, such as iron oxide, in the form of solid particulates, dispersed in a binder system. Typical castable solid propellant formulations include the particulate and reactive solid materials cured or bound into an elastic matrix or binder.
A variety of binder formulations are known for binding the particulate and solid reactive materials in an integral mass. One binder formulation comprises a polymeric composition which results in a propellant which is much more easily and safely handled than a simple mixture of the reactive solid materials themselves.
The mechanical properties of the binder formulation are critical. It is important to provide an end product which can be handled and satisfactory perform in the specific use environment. For example, a rocket motor must be easily loadable with propellant and be capable of operating in the environment encountered by the rocket motor, including operation under a variety of mechanical stresses and over a wide range of temperatures. A propellant also should be easily cleaned and de-loaded from the rocket motor if necessary, such as in demilitarizing certain rocket apparatus.
Thus, the nature of the binder can become extremely important depending upon the environments in which the resulting propellant must operate. For some propellants it may be necessary to operate under a wide range of conditions, from extreme cold to extreme heat. For other uses, temperature variation within a much more narrow range may occur. It is necessary for the final propellant formulation to have the necessary chemical properties and energy output, but also to maintain adequate mechanical properties over the range of expected operating conditions.
When a propellant or explosive is evaluated for its physical characteristics it is generally found that a propellant stress of about 100 psi is acceptable, and propellant strain in the 30 to 70% range is preferable. A binder matrix having these physical characteristics is generally acceptable for most applications.
Propellants can be cured by a chemical cross-linking reaction within a polymer binder. This process is known to provide binders, including thermosets, having the desired mechanical integrity. A common and relatively versatile chemically cured binder is obtained, for example, by the urethane reaction of a multi-functional hydroxyl terminated pre-polymer with a multi-functional isocyanate.
One of the significant disadvantages of chemically cured binder formulations, however, is that they are difficult to process and handle, although the necessary binding effect on the solid particulate reactive materials is obtained. Such chemically curable binders typically have a limited pot life, and thus must be cast into the casing, such as a projectile or rocket motor, within a short time after preparation. Otherwise, a chemically cured or chemically curing binder results, and once curing commences casting is no longer a viable option. Chemically cured binders often cure prematurely, i.e. prior to being cast, and thus an entire propellant batch can be lost due to premature curing.
An additional distinct disadvantage is associated with the essentially permanent chemical cross-linking characteristic of chemically cured binder formulations. Once the propellant is loaded in a rocket motor and cures to a chemically cross-linked product, it is very difficult to clean out the propellant. This is a distinct disadvantage when it becomes necessary to re-load a propellant, replace a propellant, remove the propellant for further processing, or demilitarize a rocket apparatus.
Propellants based on chemically cured binder formulations suffer from further disadvantages. For instance, poor interfacial mechanical properties between batches can arise, which creates difficulty in knitting one propellant batch to a previous one in a multiple-batch rocket motor even if critical propellant properties are verified prior to loading the rocket motor.
Thus, it would be advantageous to provide a binder and resulting propellant which did not rely on chemical crosslinking, yet had many of the desirable mechanical characteristics of chemically crosslinked binder formulations. Under ideal conditions, it would be desirable to provide a propellant which could be processed at elevated temperature and which solidifies and coalesces through physical, not chemical mechanisms, i.e. a propellant based on a thermoplastic elastomer binder formulation. In order to cast, recast, or remove such a propellant from a rocket motor, it would only be necessary to heat the propeliant until it becomes flowable.
Formulation of certain propellants of this latter type have been attempted. One recent effort to provide a binder having acceptable processing and mechanical properties has been to use a hybrid thermoplastic elastomer (sometimes referred to as "TPE"). The thermoplastic portion of the polymer generally takes the form of a "brittle" polymer, such as polystyrene, whereas the elastomeric portion is based on an elastomeric polymer such as polybutadiene. One such polystyrene polybutadiene TPE is manufactured by Shell Chemical under the trademark Kraton.RTM. which is structurally an "ABA" type block copolymer, i.e., an end block of polystyrene, a mid-block of polybutadiene, and another end block of polystyrene.
A binder based on the above-mentioned ABA block copolymer formulation exhibits the thermoplastic properties of characteristics of polystyrene and the elastomeric properties of polybutadiene. Thermoprocessing is therefore possible with such a material, i.e. it is possible to simply heat the material in order to make it more workable. When heated, the various blocks pool together to form a flowable and processable material. This occurs because the polystyrene is not soluble in the polybutadiene, and vice versa.
This type of block copolymer can be used in a number of commercial applications. For example, it is widely used in athletic shoes where high processing temperatures are acceptable. This type of block copolymer has also been used to coat wires, and in a number of other similar commercial applications.
One obvious drawback for energetic applications, explosives and propellants is the need to process the binder and other propellant ingredients at high temperature. The use of high temperatures to process a composition which includes materials capable of releasing explosive energies poses hazards and should be avoided. Accordingly, the acceptable processing temperature for formulation and casting of a propellant or explosive is generally much lower than the acceptable temperature for manufacturing an athletic shoe or other consumer item.
In order to address the problems of processing thermoplastic elastomer-based binders at acceptable temperatures, it has generally been necessary to provide thermoplastic elastomer-based binder formulations having significant quantities of plasticizer, such as in excess of 60 wt. %. While the addition of some plasticizer is not a concern, the addition of sufficient plasticizer to render typical thermoplastic elastomers processable at acceptably low temperatures has presented problems. One of the primary problems has been the loss of mechanical integrity of the formulation. For instance, binder formulations of thermoplastic elastomers loaded with substantial amounts of plasticizers achieve acceptable processing characteristics, while sacrificing the desired mechanical integrity of the final product. This is an undesired trade-off in critical binder properties.
Accordingly, there has been a need for thermoplastic compositions which have the desirable mechanical and chemical properties of thermoplastic elastomers and which are also easily processable at acceptable temperatures.
More particularly, the art has sought, but so far failed to provide, suitable compositions which are reasonably processable without requiring addition of excessive quantities of plasticizers to attain acceptable processing characteristics and without sacrificing desired chemical and mechanical properties. It would thus be a significant advancement in the art, particularly in energetic applications, to provide compositions and methods for producing such compositions.